tests: simplify tests for my.core.serialize a bit and simplify tox file

my/core/pytest.py

@@ -0,0 +1,22 @@
+"""
+Helpers to prevent depending on pytest in runtime
+"""
+
+from .common import assert_subpackage; assert_subpackage(__name__)
+
+import sys
+import typing
+
+under_pytest = 'pytest' in sys.modules
+
+if typing.TYPE_CHECKING or under_pytest:
+    import pytest
+
+    parametrize = pytest.mark.parametrize
+else:
+
+    def parametrize(*args, **kwargs):
+        def wrapper(f):
+            return f
+
+        return wrapper